The invention relates to a process for the preparation of N,N-diaryl-ureas by reaction of a diarylamine with an isocyanate in the presence of acid compounds.
The preparation of symmetric and unsymmetric ureas by reaction of amines with isocyanates is already known (Houben-Weyl, Methoden der organischen Chemie (Methods of Organic Chemistry), Volume VIII (1952), page 157). If necessary, this reaction is carried out in the presence of basic catalysts. In the case of reactions which proceed with difficulty, for example with sterically hindered amines or those of low nucleophilicity, such as diarylamines, drastic reaction conditions, such as extremely long reaction times, high reaction temperatures or large excesses of isocyanate, are necessary even in the presence of such basic catalysts. Side reactions cannot be excluded with such intensified reaction conditions; for example, the trimerization of the methyl isocyanate which occurs in the presence of basic catalysts greatly reduces the yield of desired urea. It is furthermore known (Chem. Ber. 115 (1982), page 919; and Chem. Ber. 117, (1984), page 1707) that reaction of the diarylamines of low reactivity is successful with isocyanates which have a strongly electron-withdrawing group, for example with (CF.sub.3 CO)NCO or SF.sub.5 --NCO. Such isocyanates are difficult to handle technically and the electron-withdrawing groups mentioned cannot subsequently be replaced by other desired substituents.